1 Basic ISDN Course Layer 2 Q-921

2 ITU-T Q.921 ISDN user-network interface – Data link layer specification

3 LAPD This Recommendation specifies the frame structure, elements of procedure, format of fields and procedures for the proper operation of the Link Access Procedure on the D-channel, LAPD.

4 Layer 2, Datalink Layer Application Presentation Session Transport Network Datalink Physical

5 Frame Types Flag Address Field Control Field Flag Frame Check Sequence Flag Address Field Control Field Flag Frame Check Sequence Layer 3 Contents Layer 2 Header

6 Frame Composition

7 Flag Sequence

8 Address Field The address field shall consist of two octets. A single octet address field is reserved for LAPB (Link Access Procedure – Balanced) operation in order to allow a single LAPB data link connection to be multiplexed along with LAPD data link connections. NOTE – The support of a LAPB data link connection within the D- channel is optional at both the network and user side.

9 Control Field The control field shall consist of one or two octets. The next slide illustrates the two frame formats (A and B), each with a control field of one or two octets, depending upon the type of frame.

10 Frame Formats

11 Information Field The information field of a frame, when present, follows the control field and precedes the frame check sequence. The contents of the information field shall consist of an integer number of octets. The maximum number of octets in the information field is defined as 260 octets.

12 Transparency A transmitting data link layer entity shall examine the frame content between the opening and closing flag sequences, (address, control, information and FCS fields) and shall insert a 0 bit after all sequences of five contiguous 1 bits (including the last five bits of the FCS) to ensure that a flag or an abort sequence is not simulated within the frame. A receiving data link layer entity shall examine the frame contents between the opening and closing flag sequences and shall discard any 0 bit which directly follows five contiguous 1 bits.

13 Frame Check Sequence, FCS

14 Numbering Convention The bits are grouped into octets. The bits of an octet are shown horizontally and are numbered from 1 to 8. Multiple octets are shown vertically and are numbered from 1 to n. The octets are transmitted in ascending numerical order; inside an octet bit 1 is the first bit to be transmitted.

15 Field Mapping Convention When a field is contained within a single octet, the lowest bit number of the field represents the lowest order value. When a field spans more than one octet, the order of bit values within each octet decreases as the octet number increases. The lowest bit number associated with the field represents the lowest order value. For example, a bit number can be identified as a couple (o, b) where o is the octet number and b is the relative bit number within the octet. The figure illustrates a field that spans from bit (1, 3) to bit (2, 7). The high order bit of the field is mapped on bit (1, 3) and the low order bit is mapped on bit (2, 7).

16 FCS, Convention An exception to the preceding field mapping convention is the data link layer FCS field, which spans two octets. In this case, bit 1 of the first octet is the high order bit and bit 8 of the second octet is the low order bit

17 Invalid Frames An invalid frame is a frame which: a)is not properly bounded by two flags; or b)has fewer than six octets between flags of frames that contain sequence numbers and fewer than five octets between flags of frames that do not contain sequence numbers; or c)does not consist of an integral number of octets prior to zero bit insertion or following zero bit extraction; or d)contains a frame check sequence error; or e)contains a single octet address field; or f)contains a service access point identifier which is not supported by the receiver. Invalid frames shall be discarded without notification to the sender. No action is taken as the result of that frame.

18 Frame Abort Receipt of seven or more contiguous 1 bits shall be interpreted as an abort and the data link layer shall ignore the frame currently being received.

19 Address Field Format The address field format shown contains the address field extension bits, a command/response indication bit, a data link layer Service Access Point Identifier (SAPI) subfield, and a Terminal Endpoint Identifier (TEI) subfield.

20 Address Field Extension The address field range is extended by reserving the first transmitted bit of the address field octets to indicate the final octet of the address field. The presence of a 1 in the first bit of an address field octet signals that it is the final octet of the address field. The double octet address field for LAPD operation shall have bit 1 of the first octet set to a 0 and bit 1 of the second octet set to 1, otherwise the frame shall be ignored.

21 Command/Response Field Bit The C/R bit identifies a frame as either a command or a response. The user side shall send commands with the C/R bit set to 0, and responses with the C/R bit set to 1. The network side shall do the opposite; that is, commands are sent with C/R set to 1, and responses are sent with C/R set to 0. The combinations for the network side and user side are shown.

22 SAPI / TEI Identifiers In conformance with HDLC (High-level Data Link Control procedures) rules, both peer entities on a point-to-point data link connection use the same Data Link Connection Identifier (DLCI) composed of a SAPI-TEI. Where SAPI and TEI conform to the definitions in the following slides to define the data link connection.

23 Service Access Point Identifier, SAPI The SAPI identifies a point at which data link layer services are provided by a data link layer entity type to a layer 3 or management entity. Consequently, the SAPI specifies a data link layer entity type that should process a data link layer frame and also a layer 3 or management entity which is to receive information carried by the data link layer frame. The SAPI allows 64 service access points to be specified, where bit 3 of the address field octet containing the SAPI is the least significant binary digit and bit 8 is the most significant.

24 SAPI

25 Terminal Endpoint Identifier It is possible to associate a TEI with a single Terminal Equipment (TE) for a point-to-point data link connection. If a TEI is not the group TEI and is not associated with any TE, that TEI is unassigned. A TE may contain one or more TEIs used for point-to-point data transfer. The TEI for a broadcast data link connection is associated with all user side data link layer entities containing the same SAPI. The TEI subfield allows 128 values where bit 2 of the address field octet containing the TEI is the least significant binary digit and bit 8 is the most significant binary digit.

26 TEI Conventions The following conventions shall apply in the assignment of these values. - TEI for broadcast data link connection: The TEI subfield bit pattern (= 127) is defined as the group TEI. The group TEI is assigned permanently to the broadcast data link connection associated with the addressed Service Access Point (SAP). - TEI for point-to-point data link connection TEI, values other than 127 are used for the point-to-point data link connections associated with the addressed SAP. The range of TEI values shall be allocated as shown below. TEI ValueUser Type 0-63 Non-automatic TEI assignment user equipment Automatic TEI assignment user equipment

27 TEI Selection Non-automatic TEI values are selected by the user, and their allocation is the responsibility of the user. Automatic TEI values are selected by the network, and their allocation is the responsibility of the network.

28 TE1 Stages

29 TE1 Assignment/De-allocation Procedure The function of this process is to: To assign a valid TEI value to a terminal via the exchange Check (by exchange) the validity of pre-installed TEI value. Enable the de-allocation of an assigned TEI value The TEI assignment is done when a terminal is plugged in for the first time and wants to communicate with the exchange. The TEI value, which is assigned, is stored in the terminal and is valid until the terminal is removed from the network.

30 TE1 Assignment/De-allocation Procedure TEI assignment is done in UI-frames using SAPI 63 and TEI 127 as address value. The exchange can check the availability of a terminal via a test routine, if the TEI is assigned the terminal will respond. Should no terminal respond after one second of monitoring time the exchange will repeat the test, if again there is no response the TEI value in question will be considered as non assigned. The TEI can be de-allocated by sending a UI (SAPI = 63 TEI = 127) TEI DEALLOCATE.

31 Layer 2 Link Status

32 TE1 Assignment Terminal TEI =127/SAPI=63 “TEI assigned, TEI=69” ISDNISDN UI

33 Automatic TE1 Selection UI frame Terminal TEI =64/SAPI=0 UI frame “ID (TEI) assigned, TEI=64” ISDNISDN

34 Broadcast ISDN SETUP(TEI=127/SAPI=0) TEI=64TEI=65TEI=70

35 TE1 Management All messages for TEI management are send in UI-frames with SAPI = 63 and TEI = 127

36 Coding

37 Control field Formats

38 Poll / Final (P/F) Bit All frames contain the Poll/Final (P/F) bit. The P/F bit serves a function in both command frames and response frames. In command frames the P/F bit is referred to as the P bit. In response frames it is referred to as the F bit. The P bit set to 1 is used by a data link layer entity to solicit (poll) a response frame from the peer data link layer entity. The F bit set to 1 is used by a data link layer entity to indicate the response frame transmitted as a result of a soliciting (poll) command.

39 Procedure for the use of the P/F bit A data link layer entity receiving an SABME, DISC, RR, RNR, REJ or I frame, with the P bit set to 1, shall set the F bit to 1 in the next response frame it transmits, as defined in the table.

40 Information Transfer (I) Format The I format shall be used to perform an information transfer between layer 3 entities. The functions of N(S), N(R) and P are independent; that is, each I frame has an N(S) sequence number, an N(R) sequence number which may or may not acknowledge additional I frames received by the data link layer entity, and a P bit that may be set to 0 or 1.

41 Example of Numbered Frames

42 Anomaly Procedure Example

43 I - Frames Used for numbered (secure) transmission of Layer 3 data

44 Supervisory (S) Format The S format shall be used to perform data link supervisory control functions such as: - Acknowledge I frames, - Request retransmission of I frames, and - Request a temporary suspension of transmission of I frames. The functions of N(R) and P/F are independent, that is, each supervisory frame has an N(R) sequence number which may or may not acknowledge additional I frames received by the data link layer entity, and a P/F bit that may be set to 0 or 1.

45 S - Frames Used for data transfer control and acknowledgement of I-frames

46 Unnumbered (U) Format The U format shall be used to provide additional data link control functions and unnumbered information transfers for unacknowledged information transfer. This format does not contain sequence numbers. It does include a P/F bit that may be set to 0 or 1.

47 U - Frames Used for unnumbered transmission of control characters, for the set-up and release of Layer 2 connections

48 Modulus Each I frame is sequentially numbered and may have the value 0 through n minus 1 (where n is the modulus of the sequence numbers). The modulus equals 128 and the sequence numbers cycle through the entire range, 0 through 127. NOTE – All arithmetic operations on state variables and sequence numbers contained in this Recommendation are affected by the modulus operation.

49 Send state Variable V(S) Each point-to-point data link connection endpoint shall have an associated V(S) when using I frame commands. V(S) denotes the sequence number of the next I frame to be transmitted. The V(S) can take on the value 0 through n minus 1. The value of V(S) shall be incremented by 1 with each successive I frame transmission, and shall not exceed V(A) by more than the maximum number of outstanding I frames k. The value of k may be in the range of 1 < k < 127.

50 Acknowledge State Variable V(A) Each point-to-point data link connection endpoint shall have an associated V(A) when using I frame commands and supervisory frame commands / responses. V(A) identifies the last I frame that has been acknowledged by its peer [V(A) - 1 equals the N(S) of the last acknowledged I frame]. V(A) can take on the value 0 through n minus 1. The value of V(A) shall be updated by the valid N(R) values received from its peer. A valid N(R) value is one that is in the range V(A) < N(R) < V(S).

51 Send Sequence Number N(S) Only I frames contain N(S), the send sequence number of transmitted I frames. At the time that an in-sequence I frame is designated for transmission, the value of N(S) is set equal to V(S).

52 Receive State Variable V(R) Each point-to-point data link connection endpoint shall have an associated V(R) when using I frame commands and supervisory frame commands/responses. V(R) denotes the sequence number of the next in- sequence I frame expected to be received. V(R) can take on the value 0 through n minus 1. The value of V(R) shall be incremented by one with the receipt of an error-free, in-sequence I frame whose N(S) equals V(R).

53 Receive Sequence Number N(R) All I frames and supervisory frames contain N(R), the expected send sequence number of the next received I frame. At the time that a frame of the above types is designated for transmission, the value of N(R) is set equal to V(R). N(R) indicates that the data link layer entity transmitting the N(R) has correctly received all I frames numbered up to and including N(R) - 1.

54 Commands and Responses The following commands and responses are used by either the user or the network data link layer entities and are represented in next Table. Each data link connection shall support the full set of commands and responses for each application implemented. The frame types associated with each of the two applications are identified in the same table. Frame types associated with an application not implemented shall be discarded and no action shall be taken as a result of that frame. For purposes of the LAPD procedures in each application, those encodings not identified in the table are identified as undefined command and response control fields.

55 Signals

56 Commands & Responses (modulo 128)

57 Information (I) Command The function of the information (I) command is to transfer, across a data link connection, sequentially numbered frames containing information fields provided by layer 3. This command is used in the multiple frame operation on point ‑ to ‑ point data link connections.

58 Set Asynchronous Balance Mode Extended, SABME The SABME unnumbered command is used to place the addressed user side or network side into modulo 128 multiple frame acknowledged operation. No information field is permitted with the SABME command. A data link layer entity confirms acceptance of an SABME command by the transmission at the first opportunity of a UA response. Upon acceptance of this command, the data link layer entity's V(S), V(A) and V(R) are set to 0. The transmission of an SABME command indicates the clearance of all exception conditions. Previously transmitted I frames that are unacknowledged when this command is processed remain unacknowledged and are discarded. It is the responsibility of a higher level (for example, layer 3) or the management entity to recover from the possible loss of the contents of such I frames.

59 Disconnect (DISC) Command The DISC unnumbered command is used to terminate the multiple frame operation. No information field is permitted with the DISC command. The data link layer entity receiving the DISC command confirms the acceptance of a DISC command by the transmission of a UA response. The data link layer entity sending the DISC command terminates the multiple frame operation when it receives the acknowledging UA or DM response. Previously transmitted I frames that are unacknowledged when this command is processed remain unacknowledged and are discarded. It is the responsibility of a higher level (for example, layer 3) or the management entity to recover from the possible loss of the contents of such I frames.

60 Unnumbered Information (UI) Command When a layer 3 or management entity requests unacknowledged information transfer, the UI unnumbered command is used to send information to its peer without affecting data link layer variables. UI command frames do not carry a sequence number and therefore, the UI frame may be lost without notification.

61 UI - Frames Used for transmission of unnumbered, unsecured layer 3 data (e.g. incoming set-up message ) and unnumbered transmission of Layer 2 messages (e.g.. TEI allocation procedure).

62 Receiver Ready (RR) Command/Response The RR supervisory frame is used by a data link layer entity to: a)indicate it is ready to receive an I frame; b)acknowledge previously received I frames numbered up to and including N(R) - 1; and c)clear a busy condition that was indicated by the earlier transmission of an RNR frame by that same data link layer entity. In addition to indicating the status of a data link layer entity, the RR command with the P bit set to 1 may be used by the data link layer entity to ask for the status of its peer data link layer entity.

63 Reject (REJ) Command/Response 1 The REJ supervisory frame is used by a data link layer entity to request retransmission of I frames starting with the frame numbered N(R). The value of N(R) in the REJ frame acknowledges I frames numbered up to and including N(R) - 1. New I frames pending initial transmission shall be transmitted following the retransmitted I frame(s). Only one REJ exception condition for a given direction of information transfer is established at a time. The REJ exception condition is cleared (reset) upon the receipt of an I frame with an N(S) equal to the N(R) of the REJ frame. An optional procedure for the retransmission of an REJ response frame is described in Appendix I.

64 Reject (REJ) Command/Response 2 The transmission of an REJ frame shall also indicate the clearance of any busy condition within the sending data link layer entity that was reported by the earlier transmission of an RNR frame by that same data link layer entity. In addition to indicating the status of a data link layer entity, the REJ command with the P bit set to 1 may be used by the data link layer entity to ask for the status of its peer data link layer entity.

65 Receiver Not Ready (RNR) Command/Response The RNR supervisory frame is used by a data link layer entity to indicate a busy condition; that is, a temporary inability to accept additional incoming I frames. The value of N(R) in the RNR frame acknowledges I frames numbered up to and including N(R) - 1. In addition to indicating the status of a data link layer entity, the RNR command with the P bit set to 1 may be used by the data link layer entity to ask for the status of its peer data link layer entity.

66 Unnumbered Acknowledge (UA) Response The UA unnumbered response is used by a data link layer entity to acknowledge the receipt and acceptance of the mode ‑ setting commands (SABME or DISC). Received mode-setting commands are not processed until the UA response is transmitted. No information field is permitted with the UA response. The transmission of the UA response indicates the clearance of any busy condition that was reported by the earlier transmission of an RNR frame by that same data link layer entity.

67 Disconnected Mode (DM) Response The DM unnumbered response is used by a data link layer entity to report to its peer that the data link layer is in a state such that multiple frame operation cannot be performed. No information field is permitted with the DM response.

68 Frame Reject (FRMR) Response 1 The FRMR unnumbered response may be received by a data link layer entity as a report of an error condition not recoverable by retransmission of the identical frame, i.e. at least one of the following error conditions resulting from the receipt of a valid frame: a)the receipt of a command or response control field that is undefined; b)the receipt of a supervisory or unnumbered frame with incorrect length; c)the receipt of an invalid N(R); or d)the receipt of a frame with an information field which exceeds the maximum established length.

69 Frame Reject (FRMR) Response 2 An undefined control field is any of the control field encodings that are not identified. A valid N(R) value is one that is in the range V(A) < N(R) < V(S). An information field which immediately follows the control field and consists of five octets (modulo 128 operation) is returned with this response and provides the reason for the FRMR response. This information field format is given in the next figure.

70 Frame Reject (FRMR) Response

71 Activation Layer 2 NTTE SABME UA Deactivated Activated

72 Deactivation Layer 2 NTTE UA DISC Deactivated Activated Deactivated Activated

73 Communication Set-up UI SABME UA DISC UA DATA (eg Layer 3 Setup)